Caption: Heart valve surgery. Transoesophageal echo (TOE) ultrasound scan of the heart of a patient during heart valve replacement surgery. The aortic valve, which is to be replaced, is at centre left. A balloon (black, centre right) is shutting off blood flow in the aorta. There are four valves in the human heart, all of which prevent the backflow of blood. Damage to the valves can lead to malfunctioning of the heart and death. If a valve cannot be repaired it must be replaced.